An U.S. Pat. No. 5,589,644 discloses a wrench 1c for measuring turning angles, referring to FIG. 1, which can detect turning angles of a coupler 1c′ of a driving head of the wrench 1c through an angle detection system 7, and calibrate fastening tightness of a work piece according to the turning angles of the wrench. The angle detection system is applicable only to the specific wrench 1c and not adaptable to other tools or changeable in terms of use positions arbitrarily, hence results in a higher production cost, and also is more complicated in structure. Thus its usability is limited.
FIG. 2 illustrates another embodiment of the wrench 1d for measuring turning angles according to U.S. Pat. No. 5,589,644 that has an angle detection system 8 to detect the turning angles of a coupler 1d′ of the driving head thereof. The angle detection system is embedded in the wrench 1d and cannot be shared with other tools, thus its applicability also is limited.